There are now signs that printed circuit board designs are getting more and more frequently. As data rates continue to increase, the bandwidth required for data transfer also causes the upper limit of the signal frequency to reach 1 GHz or even higher. Although high-frequency signal technology far exceeds the millimeter wave technology range (30 GHz), it does involve RF and low-end microwave technology.
RF engineering methods must be able to handle the strong electromagnetic field effects typically produced at higher frequency bands. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, causing unwanted crosstalk (interference and total noise) and can compromise system performance. Return loss is mainly caused by impedance mismatch, which has the same effect on the signal as additive noise and interference.
There are two negative effects of high return loss: 1. The signal reflected back to the signal source will increase the system noise, making it more difficult for the receiver to distinguish the noise from the signal; 2. Any reflected signal will basically reduce the signal quality because the input signal The shape has changed.
Although the digital system only processes the 1 and 0 signals and has very good fault tolerance, the harmonics generated when the high speed pulse rises will cause the higher the frequency, the weaker the signal. Although forward error correction techniques can eliminate some of the negative effects, part of the bandwidth of the system is used to transmit redundant data, resulting in reduced system performance. A better solution is to make the RF effect helpful rather than detract from the integrity of the signal. It is recommended that the total return loss of the digital system at the highest frequency (usually a poor data point) is -25dB, which is equivalent to a VSWR of 1.1.
The goal of PCB design is to be smaller, faster, and less costly. For RFPCBs, high-speed signals sometimes limit the miniaturization of PCB designs. Currently, the main method to solve the crosstalk problem is to perform ground plane management, spacing between wirings and reducing lead inductance (studcapacitance). The main method to reduce the return loss is to perform impedance matching. This method includes efficient management of the insulating material and isolation of the active signal and ground lines, especially between the signal lines where the state transitions and ground.
Since the interconnection point is the weakest link in the circuit chain, in the RF design, the electromagnetic properties at the interconnection point are the main problems facing the engineering design. It is necessary to examine each interconnection point and solve the existing problems. The interconnection of the board system includes three types of interconnections, such as chip-to-board, interconnection within the PCB, and signal input/output between the PCB and external devices.